Hepatitis C virus (HCV) infection is common in young injection drug users in San Francisco even though most use sterile syringes. HIV infection is relatively uncommon, and may be contained by the use of sterile equipment. We propose an epidemiological and ethnographic study to investigate this anomaly, and to study HIV and hepatitis infections in young injectors. First, we will examine HIV infection cross-sectionally in 1200 injectors aged 29 and under. We will explore risk factors for prevalent HIV infection in this needle exchange-using population. Second, we will carry out a prospective cohort study of HCV seroconversion. The principal aim of the proposed research is to estimate the seroconversion rate for HCV in the young injector population and to determine the reasons for seroconversion. In particular, we will examine the sharing of drug doses and of injection equipment other than syringes as the main reason for ongoing HCV transmission. We will accrue a cohort of 300 HCV seronegative injectors from among the 1200 persons screened and will follow them for one year. We will record HCV seroconversion prospectively. We will also identify individuals with early HCV infection, using a viral RNA amplification technology. Third, we will carry out a randomized controlled trial of a method to improve vaccination against hepatitis B virus (HBV) in young injectors. We will randomize eligible cohort members to either an accelerated vaccine schedule, which may be effective in achieving rapid immunity in young IDUs, or the standard schedule, and will study the development of immunity. Fourth, because little is known about detailed injection and sharing practices among young injectors, we will complement the epidemiological studies with an ethnographic investigation of the injecting practices of street-based youth in their natural context. We will examine how the moral economy of mutual dependency in a street-based youth culture promotes frequent paraphernalia sharing among network members, but direct needle sharing between running partners. We will explore aspects of the cultural, economic and social-status determinants of risk-taking that may explain the differential HCV and HIV infection rates.